Flathead Lake
Lake County, MontanaView mapDirections
Montana DEQ publishes the water-quality measurements for Flathead Lake. Here is what has been measured.
- Impaired · Mercury
- Public access
- Limited data
Flathead Lake in Lake County, Montana has water-quality measurements published by Montana DEQ, shown here as reported. Water clarity: 11.8 ft visibility.
Flathead Lake is a lake in Lake County, Montana. Limited fish-species data available. Public access is available with 2 public boat landings.
Source: EPA Water Quality Portal sampling records, Montana DEQ, last sampled 2012-07-11. Methodology: how these readings are compared.
Flathead Lake has 2 public boat landings, so a lakefront rental is not the only way to get a boat on the water. If you are planning to stay, you can search lake houses and cabins around Flathead Lake on Vrbo, or look for a hotel room near the lake on Booking.com or Expedia.
What Flathead Lake is good for
From access and monitoring records, not editorial picks.
- Clear water11.8 ft of visibility
- Boat launch2 documented ramps
Swimming conditions
No readingLimited data
Nutrient and algae readings are not a swimming clearance. Flathead Lake is listed impaired under Clean Water Act §303(d) (IR 5) for mercury, nutrients (phosphorus, nitrogen), pCBs. Check local advisories before you swim.
Whether Flathead Lake is safe to swim in today is a call for the Montana Department of Environmental Quality and the county health department, who post current advisories. LakeQuality does not issue swimming clearances.
Water quality measurements for Flathead Lake
These are the measurements Montana DEQ publishes for Flathead Lake, shown as reported. The condition class beside each reading is the EPA National Lakes Assessment class for this lake’s ecoregion.
Good clarity, visible to about 11.8 ft. TSI 42.
| Measure | Reading | Where it sits |
|---|---|---|
| Water clarity (Secchi depth) | 11.8ft | Measured, not scored How deep a black-and-white disk stays visible from the surface. Higher is clearer. |
| Total phosphorus | No reading The nutrient that most limits algae growth in freshwater. More phosphorus, more algae potential. | |
| Chlorophyll-a (algae) | No reading The green pigment in algae — a direct measure of how much algae is in the water. | |
| Trophic state (TSI)Carlson's 0–100 index of how much algae a lake can support, from clarity, phosphorus and chlorophyll-a. | 42 | Mesotrophic OligotrophicMesotrophicEutrophicHypereutrophic |
Moderate nutrients, good water quality
Ask about Flathead Lake
Answers come straight from the public measurements and survey records on this page.
→ Flathead Lake water level: the live USGS gauge reading
Recreation & Access
- Rollins Lakeshore DrivePublic
- Park LanePublic
Named access points in the USGS National Boat Ramps inventory, within about 3 miles. More lakes with boat ramps
Stays via Stay22
Stays near Flathead Lake
Compare lakeside cabins, hotels, and vacation rentals near Flathead Lake in Lake County, Montana — live availability and prices for your trip. Switch the map to Satellite to see the lake and how close each stay is to the water.
Top-rated places to stay near Flathead Lake
Prices for Oct 30–Nov 1, 4 adults. Change dates on the map below.
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Planning a trip? Cabins and lodging at Flathead Lake → · more Montana lake trips
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Fishing Flathead Lake
EPA Impairment Status
Flathead Lake is officially listed as impaired under Clean Water Act §303(d) in the 2021 EPA reporting cycle (IR category 5).
Causes of impairment
Designated uses not supporting: Aquatic life.
A Total Maximum Daily Load (TMDL) plan has been approved.
Source: EPA ATTAINS assessment unit MT76O003_010 · Official waterbody report
Nearby Lakes in Lake County
Frequently asked questions about Flathead Lake
Where is Flathead Lake?
- Flathead Lake is a lake in Lake County, Montana. Its water-quality measurements are published by Montana DEQ and shown on this page as reported.
What is the water quality of Flathead Lake?
- Flathead Lake in Lake County, Montana has water-quality measurements published by Montana DEQ, shown here as reported. Water clarity: 11.8 ft visibility.
Is Flathead Lake safe to swim in?
- Nutrient and algae readings are not a swimming clearance. Flathead Lake is listed impaired under Clean Water Act §303(d) (IR 5) for mercury, nutrients (phosphorus, nitrogen), pCBs. Check local advisories before you swim.
Are there boat ramps on Flathead Lake?
- Flathead Lake has 2 documented boat landings in the USGS National Boat Ramps inventory. Named access points include Rollins Lakeshore Drive, Park Lane. Public access is listed as yes.
Can I fish at Flathead Lake?
- Flathead Lake has 2 documented boat landings in the USGS National Boat Ramps inventory. Seasons, size and bag limits, and whether you need a license are set by Montana's fish and wildlife agency, so check its current regulations before you go.
Explore Flathead Lake across LakeQuality
Depth, room, public access, and the state wake-sport rules
Species activity outlook by water temperature and season
Where to stay at Montana's top lakes, each with its readings
Montana's cleanest lakes, ranked on agency measurements
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Where this data comes from
2 measurements from 1 monitoring station, 2012. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- North American Lake Management Society
NALMS2 samples · 2012
Monitoring station identifiers (1)
EPA condition classes from the National Lakes Assessment (EPA 841-R-24-006, Table 6-3), applied by ecoregion. See where the data comes from and data sources.
Other records on this page
Impairment status from EPA ATTAINS 303(d) database
Named boat-access points from the USGS National Boat Ramps inventory